Effects of Artificial Small Defect and Corrosive Environment on Torsional Fatigue Strength of High Strength Spring Steel

Torsional fatigue tests were conducted for spring steels whose Vickers hardness were 430, 480, 550, and 620. The effects of corrosive environment and surface artificial pits on the fatigue strength were studied. The fatigue strength of smooth specimens was maximum at 550HV for fatigue in air and 480HV for corrosion fatigue. Intergranular fracture surfaces were observed on the fatigue fracture surface corresponded to the decrease in the fatigue strength of the high-hardness materials. The fatigue fracture of smooth specimens was generated from shear crack for fatigue in air and from corrosion pit in corrosion fatigue. The stress intensity factor for crack initiation from corrosion pit was nearly the same for all materials. The fatigue strength reduction factor was constant under fatigue loading in air, while decreased with increasing fatigue life and hardness in corrosion fatigue. For the case of long of the material with 620HV in corrosive environment, a significant influence of the artificial pit on the fatigue strength was not seen. Many small cracks were observed at the sites except for artificial pit on the surface of the 620HV specimens fatigued at low stress levels.